1. Field of the Invention
The invention relates to a fuel cell separator and production method therefor and, more particularly, to a structure of a surface-treated layer of a metal separator for a solid polymer electrolyte type fuel cell.
2. Description of the Related Art
A solid polymer electrolyte type fuel cell battery is formed by stacking modules each of which is formed by stacking at least one cell made up of a membrane-electrode assembly (hereinafter, referred to as “MEA”) and a separator.
Each MEA is made up of an electrolyte membrane formed by an ion-exchange membrane, an electrode (anode) formed by a catalyst layer that is disposed on a surface of the electrolyte membrane, and an electrode (cathode) formed by a catalyst layer that is disposed on another surface of the electrolyte membrane. Normally, a diffusion layer is provided between the MEA and the separator. The diffusion layer facilitates the diffusion of a reaction gas into the catalyst layer. The separator has a fuel gas channel for supplying a fuel gas (hydrogen) to the anode, and an oxidizing gas channel for supplying an oxidizing gas (oxygen, or air in ordinary cases) to the cathode. The separator forms a passageway of electrons between adjacent cells.
Terminals (electrode plates), insulators, and end plates are disposed on two opposite ends of a cell stack in the cell stacking direction. The cell stack is clamped in the cell stacking direction, and is fixed through the use of fastener members (e.g., tension plates) that extend outside the cell stack in the cell stacking direction, and also through the use of bolts and nuts. In this manner, a stack is formed. On the anode side of a solid polymer electrolyte type fuel cell, a reaction occurs in which hydrogen is separated into hydrogen ions (protons) and electrons. The hydrogen ions migrate through the electrolyte membrane to the cathode side. On the cathode side, the hydrogen ions participate in a reaction with oxygen and electrons (i.e., electrons produced on the anode side of the adjacent MEA come to the cathode through the separator, or electrons produced on the anode side of the cell disposed at an end of the cell stack come to the cathode of the cell at the opposite end via an external circuit), thereby producing water.Anode side: H2→2H++2e−Cathode side: 2H++2e−+(½)O2→H2O
Since the separators need to have electrical conductivity, separators are normally formed of a metal, carbon, or an electrically conductive resin, or are formed by a combination of a metal separator and a resin frame. Carbon separators and electrically conductive resin separators are chemically stable and therefore maintain electrical conductivity even during contact with acid water. However, due to a strength requirement of bottom surfaces of channels formed in separators, the carbon separators and electrically conductive resin separators need to have relatively great thickness, thus resulting in an increased stack length. In contrast, the metal separators, having relatively high strength, can be made relatively thin despite grooves and ridges being formed to provide channels. Thus, the stack length can be reduced. However, corrosion by acid water becomes a problem leading to a reduced electrical conductivity and a reduced output. That is, to adopt metal separators, it is necessary that the metal separators be able to maintain good electrical conductivity and good corrosion resistance for a long period.
As a related-art technology, Japanese Patent Application Laid-Open Publication No. 2001-93538 discloses a technology in which a surface of a substrate (stainless steel) of a metal separator of a fuel cell is provided with an electrically conductive film and an acid-resistant film of a metal material different from that of the substrate.
However, the conventional metal separator has a problem of increased cost since the acid-resistant film and the substrate are formed from different metal materials.